This invention relates to a gas turbine control system, and more particularly to a gas turbine control system suitable for use in a composite electric power generating plant to improve its load follow-up characteristic.
In recent steam electric power generating plants, as the cost of the fuel has increased, the request for improving the load follow-up characteristic including start and stop of the plants has become remarkable for the purpose of improving thermal efficiency and attaining middle load. As a solution of this problem a composite electric power generating plant comprising a combination of a gas turbine and a steam plant has been developed.
In a gas turbine utilized in such composite electric power generating plant, for the purpose of improving the overall thermal efficiency of the plant, it has been considered to increase the exhaust temperature of the gas turbine and the quantity of steam generated by a waste heat recovery boiler so as to increase the output of the steam turbine. For example, a system has been known in which the inlet stationary blades of a compressor installed before a gas turbine are made variable so as to decrease the quantity of air flowing into the gas turbine at a partial load thereby increasing the exhaust temperature of the gas turbine.
FIG. 1 is a block diagram showing a prior art gas turbine of a composite electric power generating plant of the type described above. FIG. 1 shows a prior art system of increasing the exhaust temperature of a gas turbine supplied with compressed air from a compressor provided with inlet variable stationary blades.
In FIG. 1, inlet variable stationary blades 2 are provided for the inlet 1 of an air compressor 3 so as to control the quantity of air flowing into the compressor. The air compressed by compressor 3 is sent to a burner 4 for burning fuel 6, the flow quantity thereof being adjusted with a fuel adjusting valve 5. High temperature gas formed as a result of combustion of the fuel is sent to a gas turbine 8 to drive an electric generator 10. Exhaust gas from the gas turbine 8 is sent to a waste heat recovery boiler 11 to heat water fed to the boiler 11 so as to generate steam which is sent to a steam turbine, not shown. The steam turbine is coupled to generator 10 or separate generator, not shown.
FIG. 2 is a block diagram showing a prior art gas turbine control system applicable to the gas turbine 8 shown in FIG. 1. The control system shown in FIG. 2 is constituted by a start control circuit 11, a speed/load control circuit 12, a temperature control circuit 13, and an inlet variable stationary blade control circuit 14. Fuel control signals from these control circuits 11, 12, and 13 are applied to a low value priority circuit 15 and their minimum value is sent to a fuel adjusting valve control circuit 16, the output signal thereof being sent to the fuel adjusting valve 5 to control its degree of opening. The degree of opening of the fuel adjusting valve 5 is fed back to the fuel adjusting valve control circuit 16 to act as a feedback signal 17. An adder 20 adds together a rated number of revolution bias from a bias setter 18a and a load signal from a load setter 18, and subtracts an actual number of revolutions of the gas turbine 8 detected by a revolution rate detector 19 from the resulting sum, and a difference signal thus calculated is applied to the speed/load control circuit 12. An adder 23 calculates a difference between an output signal from a temperature setter 21 generating a signal corresponding to the discharge pressure of compressor 3 and an output signal from a discharge gas temperature detector 22 for applying a difference signal to the temperature control circuit 13. Furthermore, an adder 26 calculates a difference among an output signal of another temperature setter 24 which sets the temperature of the exhaust gas of the gas turbine, the output signal from the discharge gas temperature detector 22 and a negative bias from another bias setter 25 which sets the negative bias for the purpose of preventing interference of the temperature control circuit for supplying difference signal to the inlet variable stationary blade control circuit 14, and the output thereof is sent to an inlet variable stationary blade position control circuit 27 for operating the inlet variable stationary blades 2. The position of these blades is fed back to the inlet variable stationary blade position control circuit 27 for positioning the blades.
With this control system, during starting of the gas turbine 8, the start control circuit 11 generates fuel control signals necessary to ignite and accelerate the gas turbine. However, since the rate of revolution of the gas turbine 8 is small and the temperature of the exhaust gas of the gas turbine is also low, the temperature control circuit 13 produces a fuel control signal of the maximum value. Consequently, the low value priority circuit 15 selects the signal from the start control circuit 11 so as to supply this signal to the fuel adjusting valve control circuit 16 for controlling the quantity of fuel.
When the revolution rate of the gas turbine increases near the rated number of revolutions per minute, the control is transferred from start control circuit 11 to speed/load control circuit 12. As the gas turbine and the generator 10 are brought into synchronism and connected to an electric power system, not shown, to share its load, the temperature of the exhaust gas of the gas turbine 8 increases. Then, the temperature control circuit 13 operates to limit the quantity of fuel.
The inlet variable stationary blades 2 are controlled by the inlet variable stationary blade position control circuit 27 to maintain the temperature of the exhaust gas temperature of the gas turbine 8 at a high value independently of the start control circuit 11, the speed/load control circuit 12 and the temperature control circuit 13.
FIG. 3 is a graph showing the relation between gas turbine output controlled by the gas turbine control system shown in FIG. 2 and the exhaust gas temperature. In FIG. 3, a dotted line A shows a temperature limit curve obtained by the temperature control circuit 13. The exhaust gas temperature varies along a thick solid line. The reason that this curve slopes downwardly toward the right lies in that the inlet temperature of the gas turbine is maintained at a constant value for preventing damage of the burner, and the first stage nozzles of the gas turbine caused by excessive temperature rise. Solid line B represents a temperature limit curve effected by the inlet variable stationary blade control circuit 14 and the difference between broken line A and solid line B corresponds to the magnitude of the bias signal provided by bias setter 25.
While the gas turbine is running under a partial load, since the inlet variable stationary blades are held at the minimum opening position, these blades are not influenced by the inlet variable stationary blade control circuit 14 so that an exhaust gas temperature characteristic as shown by a thick line section I-II can be obtained. As the output of the gas turbine 8 is increased, the quantity of air flowing into the gas turbine 8 is controlled by the inlet variable stationary blades 2 under control of the inlet variable stationary blade control circuit 14 so that the exhaust gas temperature would be maintained at a high value. As the output of the gas turbine is increased further, the exhaust gas temperature would be controlled as shown by sections II-III-IV. Meanwhile, the opening of the inlet variable stationary blades 2 is gradually increased with increase in the gas turbine output and these blades 2 are fully opened at position IV. As the gas turbine output is increased further, the exhaust gas temperature rises along a curve C showing a running characteristic of a gas turbine not equipped with the inlet variable stationary blades 2. As a consequence, when the exhaust gas temperature reaches a cross point V between broken line A and curve C, the temperature control circuit 13 operates to limit the quantity of fuel, thus reaching the maximum output.
As above described, with the prior art turbine control system shown in FIG. 2 and applicable to a gas turbine driven by a compressor having inlet variable stationary blades, as the load increases when the gas turbine exhaust temperature is increased by a fuel increase demand signal from the speed/load control circuit 12, the degree of opening of the inlet variable stationary blades 2 is increased to increase the air quantity so as to prevent the exhaust gas temperature from rising above a predetermined tempera-ture, thereby maintaining the exhaust gas temperature at a constant value. With this system, however, the temperature tends to exhibit a transient rise. Moreover, since the inlet variable stationary blades are operated after temperature rise, the load follow-up characteristic is poor. In other words, the response delay between sensing of the rise of the gas turbine exhaust temperature and opening of the inlet variable stationary blades causes a transient rise of the exhaust gas temperature as well as degradation of the load follow-up characteristic. Moreover, as the exhaust gas temperature is controlled by controlling the position of the inlet variable stationary blades, the quantity of air flowing into the gas turbine varies thus causing a difference between the load instruction signal from the speed/load control circuit 12 and the actual output signal of the gas turbine, which degrades the control characteristic.